some Unsequence files never be merged to higher level or Sequence folder

In rel/0.12, if we use level compaction management and enable level compaction for unsequence files, the system will not compact the unsequence files to the top level. Instead, it will merge the files that one level lower than the top to sequence file. During the merge process, the files will be set merging, so we don't need to set it before the merge.